1. Field of the Invention
The present invention relates to a welding material and a fuel tank thereby welded. More particularly, it relates to a welding material employing a specific modified polyethylene resin composition which is excellent in adhesion to polyethylene resins, various polyamide (PA) resins such as nylon 6,6, various hydroxyl group-containing resins such as an ethylene/vinyl alcohol copolymer (EVOH), various polyester resins such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), various engineering plastics such as polyacetal (POM), polyphenylene ether (PPE) and polyphenylene sulfide (PPS), various metals such as iron, aluminum, copper and tin, and various alloys, and a fuel tank thereby welded.
2. Discussion of Background
In recent years, various components of automobiles have been made of plastic and practically used for the purpose of weight reduction. Among them, a fuel tank is a large size component, and if it is made of plastic, such will contribute to weight reduction of the vehicle itself. Accordingly, a fuel tank is now being changed from one made of metal to one made of plastic (the following description will be made mainly with respect to a plastic fuel tank).
Such a plastic tank has a multilayered structure having a fuel permeation-preventing layer made of e.g. a polyamide resin (hereinafter referred to as PA) or an ethylene/vinyl alcohol copolymer (hereinafter referred to as EVOH) sandwiched between outer and inner layers made of e.g. a high density polyethylene resin. However, the high density polyethylene resin is poor in adhesion to PA or EVOH, and accordingly, it is common to employ a polyethylene resin modified with an unsaturated carboxylic acid such as maleic anhydride to bond the outer and inner layers to PA or EVOH.
The present applicant et al. have proposed such a plastic fuel tank employing e.g. a high density polyethylene resin, EVOH and a modified polyethylene resin, and an adhesive resin composition useful therefor, for example, in WO2002079323A1 (Japan Polyolefins Co., Ltd.), U.S. Pat. No. 5,643,997 (Showa Denko), U.S. Pat. No. 5,902,655 (Showa Denko), JP-A-2006-176717 (Japan Polyethylene Corporation) and JP-A-2006-176718 (Japan Polyethylene Corporation).
Further, also in JP-A-2002-264669 (TI Group), a fuel tank having a structure comprising a high density polyethylene layer, a binder layer and an EVOH layer has been proposed.
Further, to a plastic fuel tank body, it is necessary to attach various attachments such as a fuel cut-off valve, a fuel vapor recovery valve, a fuel shut-off valve, a valve cover, a connector, a pipe joint, a fuel line and a fuel sender. Such various attachments are made of a polyamide resin, a polyacetal resin, etc. and thus are poor in adhesion or weldability to a fuel tank body made of a high density polyethylene. Accordingly, at the joint area, a modified polyolefin resin will be disposed. Further, various proposals have been made to prevent fuel diffusion by providing a material (a barrier material) having a fuel permeation preventing property, such as a polyamide resin or an ethylene/vinyl alcohol copolymer to cover the modified polyolefin resin disposed at the joint area between the attachment and the fuel tank (e.g. JP-A-2000-008981 (Toyoda Gosei), JP-A-2002-370759 (Ube Kosan), JP-A-2003-072399 (Kyosan Denki), JP-A-2004-293324 (Toyoda Gosei), JP-A-2003-246028 (Ube Kosan) which corresponds to USP 20030037831, and JP-A-2005-193650 (Biolax)).
Thus, to a plastic fuel tank body or to an attachment such as a fuel sender itself, it is possible to impart a fuel permeation preventing property to some extent by including a resin layer having a barrier property in such a multilayer structure.
However, diffusion of a fuel occurs also from the joint portion (the welded portion) of the tank and the attachment, and such fuel diffusion from the joint portion (the welded portion) tends to be a non-negligible amount. Accordingly, in order to reduce the fuel diffusion amount from the entire tank portion, it is necessary to lower the fuel permeation amount from this joint portion.
However, it is widely known that as a general nature of a polyethylene resin, as the density becomes high, the stress cracking resistance or creep rupture resistance tends to deteriorate, and such a tendency is in opposite correlation with the above-mentioned fuel permeation preventing performance. Besides, a direct fuel-injection engine-mounted car has a system wherein a part of the fuel returns to the tank, whereby the fuel temperature in the tank may rise to a level of from 60 to 80° C., and accordingly, each of the stress cracking resistance, creep rupture resistance and fuel permeation preventing performance of the polyethylene resin tends to be accelerated towards deterioration. Further, when contacted with gasoline or gasohol, the polyethylene layer and the adhesive layer tend to swell, whereby the interlaminar bond strength between the polyethylene layer and the polyamide layer tends to deteriorate, thus leading to a problem such as delamination or cracking.
Accordingly, in order to utilize such a material as a constituting material for a component for a fuel tank, as an important safety component, it can hardly be said to have a sufficient performance secured from the viewpoint of safety.
Further, a modified polyolefin resin to be used at the joint surface, is required to have a firm bond strength to a material (barrier material) having a fuel permeation preventing performance such as a polyamide resin or an ethylene/vinyl alcohol copolymer, and a firm welded (fused) strength to the resin of the tank body, and further desired to be a balanced material to sufficiently satisfy various performances such as fuel permeation preventing performance and creep rupture resistance (long-term performance). However, the above-mentioned conventional modified polyethylene resins or welding materials did not fully satisfy such performances.